40 MORPHOLOGY AND EVOLUTION
The top of the cranium lies well above the supra-orbital margins: and on the side of the skull the parietal articulates with the malar bone, thus separating the frontal and alisphenoid.
The tympanic annulus is transversely short, and it is widely open laterally. It overlaps the tympanic bullse, in the possession of which the Platyrrhini agree with the Lemuroidea and Tarsioidea, and differ from the Old World Monkeys and Apes. The internal carotid artery passes through a foramen between the annulus
The bony palate is short, with a variable posterior nasal spine. The anterior palatine foramina are well-marked, but the posterior foramina are minute. The external pterygoid plates are large, but the internal plates are small, with variable hamular processes.
The width between the ascending rami of the mandible is greater than in the Lemurs as the result of the more capacious cranial cavity. The ramus aseendens is wide and deep, particularly in the Howling Monkeys with expanded hyoid bones. The coronoid process is recurved, and the symphysis menti slopes downwards and backwards.
In the interior of the skull the chief points to note are the extension of the cranial cavity, the petrosal has a cerebellar fossa of variable depth, and the tentorium is partly ossified. The posterior clinoid processes are well developed.
Teeth.—The dental formula is 11 C J PM f M -f in the Eapalidse, and I f G J PM 1 M § in the Cebida^ so, besides differences in the characters of the teeth there